This invention relates to a cryogenic turbo-expander having a rotary shaft with anti-friction bearings which carries a turbine wheel and an energy dissipating means.
By the term xe2x80x9ccryogenic turbo-expanderxe2x80x9d as used herein is meant a turbo-expander operable to create a temperature below minus 20xc2x0 C., preferably below minus 100xc2x0 C.
The energy dissipating device is typically a compressor wheel. The rotary shaft typically has two axially spaced lubricated bearing means. The lubricant is supplied in the form of a mist (i.e. in divided form), to a passage along the shaft which communicates with both bearing means.
Cryogenic turbo-expanders operate at very high rotary speeds of at least 25,000 revolutions per minute. A rotary speed of about 30,000 to 50,000 revolutions per minute is typical. Such high speeds result in a considerable generation of heat at the bearings. As a result, the consumption of lubricating oil is undesirably high. Not only does a high consumption of lubricating oil add to the cost of operating the machine, it also has the consequence that a particularly large lubricating oil reservoir is required, therefore adding appreciably to the size of the machine.
It is an aim of the present invention to provide a cryogenic turbo-expander having a reduced consumption of lubricating oil in comparison with the machine described above.
According to the present invention there is provided a cryogenic turbo-expander having a rotary shaft which carries a turbine wheel and carries or is associated with an energy dissipating means and which extends axially through a sleeve, first race means surrounding the shaft and housing first bearing means for the shaft, second race means surrounding the shaft and housing second anti-friction bearing means for the shaft, the first and second bearing means being axially spaced from one another, wherein each race means has an aperture therein extending from an outer to an inner surface thereof communicating with a lubricating oil passage extending from an outer surface to an inner surface of the sleeve and wherein both race means are arranged so that spent lubricating oil can pass therefrom to a common drain damage, the cryogenic turbo-expander additionally including a reservoir for lubricating oil communicating with the aperture in the first race means via an intermittently-operable metering pump and with the aperture in the second race means via an intermittently operable oil metering pump.
A cryogenic turbo-expander according to the invention is able to be operated with a reduced lubricating oil consumption in comparison with the machine described above. This result may be attributed to the fact that the lubricating oil is able to be supplied directly to both bearing means without travelling along the shaft and hence is supplied only intermittently but preferably is undivided form.
Both bearing means are preferably of an anti-friction kind.
Although it is possible for the passages through the sleeve to have a common inlet it is preferred that the passage communicating with the aperture in the first race be separate from the passage communicating with the aperture in the second sleeve.
The first and second oil metering pumps preferably inject lubricant into both race means at predetermined times so as to lubricate the bearings. Typically, lubricating oil is injected into both bearings 6 to 10 times per hour. The first and second oil metering pumps may additionally or alternatively be adapted to respond to signals from temperature sensors in the respective races. In this way, the creating of excessive temperatures in the races may be avoided.
The oil metering pumps are preferably both of a piston kind and are preferably both actuated by a solenoid.
The energy dissipating means is preferably a compressor wheel but may alternatively be any high speed braking device (for example, an eddy current brake or a frictional brake wheel) or a high frequency electrical generator.
A cryogenic turbo-expander according to the invention is particularly suited for use in a cryogenic air separation plant, for example a nitrogen generator.